In nature, only bound states of quarks (called hadrons) exist. If quarks are created e.g. in a high-energy experiment, they will quickly form hadrons. This process is called hadronisation or fragmentation.
Since the coupling constant (i.e. the strength) of the strong force is not small compared to typical quantities in this process, this transition cannot be calculated (e.g. by using perturbation theory). Instead, one has to rely on models. A particular sucessfull model was suggested by B. Andersson, T. Sjöstrand et al. and is called the Lund model (since it was developed at the University of Lund ). Other approaches are e.g. Cluster fragmentation, etc.
Suppose that a quark - anti-quark pair has been created e.g. in an electron-positron annihilation. They move apart back-to-back, i.e. on a straight line. Because of the self-interaction of the gluons, the field-lines will not spread out as in the case of the electro-magnetic interaction but form a small tube (called string ) connecting the quarks. As the quarks move outwards, more and more energy is stored in the field.When enough energy is stored in the string, new quark - anti-quark pairs can be created. This process is repeated as long as sufficient energy is available. Thus the system breaks into smaller and smaller pieces until finally only hadrons remain. This simplified picture is illustrated in the figure below